1. Field of Invention
The present invention relates generally to a combined reverse osmosis/continuous electrodeionization water treatment system and, more particularly, to a combined reverse osmosis/continuous electrodeionization water treatment system for producing high-purity water.
2. Description of the Prior Art
The requirement for high-purity water with particular properties has evolved in several industries including the electronics industry, the power industry, and the pharmaceutical industry. The water purity requirements of the semiconductor industry are among the most stringent of any industry. Typically these applications require treatment of a source water supply (such as a municipal water supply) to reduce the level of contaminants. High-purity water processing procedures and the hardware required for carrying out the processing are complex and expensive. A current high-purity water specification is available in the ASTM D 5127-99 standard for electronics and semiconductor industry.
Ion exchanging resins have been used to produce deionized water. Other well known processes that can be used for water purification include distillation, electrodialysis, reverse osmosis, and liquid chromatography. These ion exchanging resins generally require chemical regeneration. On-site ion exchange regeneration requires aggressive chemicals that are dangerous to handle. Removal of the spent chemicals must be dealt with in a manner that is safe for the environment. In this respect, attention has been drawn in recent years to a self-regenerating type deionizing apparatus. To avoid the use of aggressive chemicals, a deionizing function of the ion exchanging resins and an electrodialysis function of ion exchange membranes are combined in an electrodeionization apparatus to obtain high-purity deionized water without chemical regeneration (U.S. Pat. No. 6,274,019). Electrodeionization is a water purification technique that utilizes ion exchanging resins, ion exchange membranes, and electricity to deionize water (for a more detailed discussion see Wilkins, F. C., and McConnelee, P. A. “Continuous Deionization in the Preparation of Micro-electronics Grade Water”, Solid State Technology, pp 87-92, August 1988). Electrodeionization is differentiated from electrodialysis by the presence of ion exchange resin in the purifying compartments. Illustrative of other prior art attempts to use the combination of electrodialysis and ion exchanging resins to purify saline from are described in U.S. Pat. Nos. 2,796,395; 2,947,688; 2,923,674; 3,014,855; 3,384,568; and 4,165,273.
The use of electrodeionization is disclosed in U.S. Pat. Nos. 2,689,826; 2,815,320; 3,149,061; 3,291,713; 3,330,750; and many others. A commercially successful electrodeionization apparatus and process is described in U.S. Pat. No. 4,632,745. The basic repeating element, called a cell pair, consists of a purifying compartment, bounded on each side by an anion membrane and a cation membrane, which is filled with a mixed bed ion exchanging resin, and a concentrating compartment (see Wilkins and McConnelee). The feedwater entering the electrodeionization apparatus is separated into at least three parts. A small percentage flows over the electrodes, a majority of the feed water passes through the purifying compartment and the remainder passes along the concentrating compartments. Under the influence of D.C. Potential, ions in the purifying compartment are transferred into the adjacent concentrating compartment. Ions entering the resin-filled purifying compartment transfer through the resin and the ion exchange membranes in the direction of the electrical potential gradient, into the concentrating compartment (See Liang, L. S., Wood, J., and Hass W., “Design and Performance of Electrodeionization System in Power Plant Applications”, Ultrapure Water pp, 41-48, October 1992). As a result, ions in the water will become depleted in the purifying compartments and will be concentrated in the adjacent concentrating compartments. The third stream is the electrode stream that sweeps past the electrodes removing gases from electrode reactions as it flows. The percentage of the incoming feedwater that becomes purified product is referred to as the recovery of the system. In conventional electrodeionization systems with reverse osmosis product as feed, the concentrate stream can typically be recirculated to obtain recoveries in the range of 80 to 95% (see Liang et al.). U.S. Pat. No. 6,193,869 discloses the use of modular system design.
The power supply may be a constant current or a constant voltage power supply. Presently, electrodeionization apparatuses typically operate using a constant voltage power supply, in which the current is varied to maintain a constant voltage. Unfortunately, it has been observed that the electrical impedance of electrodeionization apparatuses increases with the age of the module. This impedance increase means that as the electrodeionization apparatus ages, the current passing through the apparatus decreases when powered with a constant voltage power supply (as described in U.S. Pat. No. 6,365,023). This results in the poor treated water quality from the electrodeionization apparatus. Similarly, a new electrodeionization apparatus having low impedance and run at a constant voltage can produce a very high current. Further, scaling of electrodeionization apparatus can be a problem where there is more current than necessary applied to the apparatus. Impedance of an electrodeionization apparatus increases with decreasing temperature. As a result, the risk of scaling may be low in winter. U.S. Pat. No. 6,365,023 suggests the use of constant-current power supply. Ionic removal is accomplished here by supplying a constant electrical current in the range of about 1.5 to 15 times a theoretical minimum current.
In electrodeionization devices a gasket positioned between anion and cation exchange membranes forms purifying compartments. U.S. Pat. Nos. 4,632,745; 4,747,929; 4,925,541; 4,931,160; 4,956,071; and 5,120,416 describe gasket design in electrodeionization apparatus. A need also exists for a gasket that assures good fluid flow and electrical current distribution and that has a low overall pressure drop for fluid flow (see for example U.S. Pat. No. 6,123,823).
U.S. Pat. No. 4,925,541 discloses a plate and frame electrodeionization apparatus and method. Plate and frame apparatuses are large in size and typically suffer from leaks because of the difficulty in sealing large vessels. Spiral-wound modules (U.S. Pat. No. 5,376,253 and Rychen P., Alonso S., and Alt H. P., “High-purity Water Production with the Latest Modular Electrodeionization Technology”, Ultrapure Water Europe, Amsterdam, 1996) and helical modules (U.S. Pat. No. 6,190,528) are also available.
The ion exchanging materials are commonly mixtures of cation exchanging resins and anion exchanging resins (e.g. U.S. Pat. No. 4,632,745), but alternating layers of these resins have also been described (e.g. U.S. Pat. Nos. 5,858,191 and 5,308,467). Because of their ability to exchange counter-ions, ion exchange resins are electrically conductive (Heymann E., and O'Donnell I. J., Journal of Colloid Science, Volume 4, pp 395, 1949). The resin-filled purifying compartments facilitate ion transfer along contiguous ion exchange beads by creating a low resistance electrical path, even in a highly purified solution with high resistivity (see Griffin C., “Advancements in the Use of Continuous Deionization in the Production of High-purity Water”, Ultrapure Water, pp 52-60, November 1991). A path is developed through the ion exchange resin beads that is much lower in electrical resistance than the path through the surrounding bulk solution, thereby facilitating removal of ions from the device (see Ganzi G. C., “The Ionpure™ Continuous Deionization Process: Effect of Electrical Current Distribution on Performance”, Presented at the 80th Annual AIChE Meeting, Washington D.C., November 1988). Strongly dissociated ion exchanging resins have specific electrical resistances of order of magnitude about 100 ohm-cm, i.e., about the same as an aqueous solution containing about 0.1 gram-equivalent of sodium chloride per liter. U.S. Pat. No. 5,593,563 discloses the use of electron conductive particles such as metal particle and/or carbon particles in the cathode compartment. U.S. Pat. No. 5,868,915 discloses the use of chemical, temperature, and fouling resistant synthetic carbonaceous adsorbent particles (0.5-1.0 mm diameter) in either electrolyte compartments, purifying compartments, or concentrating compartments. It is important to note that the presence of gases, poor flow distribution, low temperature and/or low conductance liquids within the electrolyte compartments may be detrimental to electric current distribution, thereby reducing the efficiency of deionization.
Undesirably, where mixed bed ion exchanging materials are used, the thickness of the purifying compartments must be necessarily thin to maximize the transport efficiency of impurity ions through the resins to the membranes (U.S. Pat. No. 6,197,174). Thinner diluting compartments dictate higher manufacturing cost. U.S. Pat. No. 4,636,296 discloses an electrodeionization apparatus containing alternating layers of anionic and cationic exchanging resins to mitigate this problem. U.S. Pat. No. 6,197,174 discloses the use of one mixed bed phase of anion and cation resins and at least one single phase, adjacent to the mixed bed phase. U.S. Pat. No. 6,156,180 discloses the use of a continuous phase of a first ion exchanging resin material containing therein a dispersed phase of clusters of a second ion exchanging resin material in the purifying compartments. This arrangement allows an increase in the thickness and size of the purifying compartments thereby permitting more resin to be placed in the purifying compartments and decreasing the necessary membrane area for a given flow rate.
When uniform-bead size resins were placed in the purifying compartments, increased ion exchange rate and accordingly better salt removal was found (see Griffin). This is due to an increase in the resin surface area and also due to an effective increase in the amount of resin active in the electrical circuit within the system. U.S. Pat. No. 5,308,466 discloses the use of low crosslinked ion exchange resin or membrane to lower the resistance of the resin or membrane. Such resins or membranes have greater interstitial water content, a greater pore size, and a decreased charge density as compared to resins and membranes having higher degrees of crosslinking. U.S. Pat. No. 5,858,191 discloses the use of Type II anion exchanging resin material, alone or with Type I anion exchanging resin material, in anion permeable membranes and/or resins to improve the electric current distribution, degree of resin regeneration, and deionization performance. The use of doped cation exchanging resin and Type I anion exchanging resin materials in the purifying compartments to reduce the difference in conductivity between the alternating layers is disclosed in U.S. Pat. No. 6,284,124. U.S. Pat. No. 6,312,577 discloses the use of macroporous ion exchanging resins that are both highly crosslinked and have a high water content. This system provides an improved removal of weakly ionized ions, particularly silica.
When ions are readily present in the feedwater, charge will pass through the purifying compartment as ions migrate into the concentrating compartment. But as these ions are removed, a point will be reached within the electrodeionization system where insufficient ions are available to carry the charge being generated at the electrodes. The resistance across the cells will substantially increase, resulting in an increase in voltage. The voltage differential across the purifying chamber will increase until it is sufficient to split water into its H+ ions and OH− ions (see Byrne W., Encyclopaedia of Water Treatment, Volume X: EDR & EDI, Version U 1.0, Wes Byrne and the Company for Educational Advancement (CEA), 1999). In electrodeionization apparatus, H+ ions and OH− ions are formed by dissociation of the water to continuously regenerate the ion exchanging resins filled in the purifying compartments so that the electrodeionization apparatus can efficiently deionize water. The high electric voltage in the dilute compartment not only splits water, but also destroys some of the low molecular weight organics that pass through the preceding reverse osmosis system (Auerswald, D., “Optimising the Performance of a Reverse Osmosis/Continuous Electrodeionization System”, Ultrapure Water, pp 35-52, May/June 1996). Electric current more than the theoretical amount required to discharge ions from feedwater is supplied to cause dissociation of water in the purifying compartments so as to continuously regenerate the ion exchanging resins. The passage of 96, 500 coulombs (one Faraday) causes the transfer of one chemical equivalent of a salt theoretically.
It has been shown (see Glueckauf E., “Electro-deionisation Through a Packed Bed”, British Chemical Engineering, pp 646-651, December 1959) that the mechanism of ion removal from purifying compartments to adjacent concentrating compartments involves the diffusion of ions to the resin phase and subsequent electrical conduction within the resin phase to the bounding membranes of the purifying compartment. In order to achieve high rates of ion removal, the cation exchanging resin should be predominantly in the hydrogen form and the anion exchanging resin should be predominantly in the hydroxide form. At the end of the purifying compartments, where water is relatively free off ions, splitting of water occurs in the electric field. This generates hydrogen and hydroxyl ions. The creation of H+ ions and OH− ions from water splitting allows the resins to remain in the hydrogen and hydroxide forms. Moreover, the resins in the regenerated forms can react with weakly ionized species, allowing transfer of the species that would not otherwise occur (as described by Ganzi).
The random nature of mixtures of cation and anion exchanging resins tends to cause some portion of the resins to be regenerated to a needlessly high degree and others inadequately regenerated. The achievement of a uniform distribution of water splitting is a more difficult problem and much effort has gone into designing structures that achieve this (for examples see U.S. Pat. Nos. 6,241,867; 5,858,191; 5,868,915 and 5,308,467).
Scaling has been found to occur in localized regions of electrodeionization apparatus, and particularly those where high pH is typically present. It is believed that the pH at the boundary layer increases with current. Therefore, the current needs to be maintained at a sufficiently low level to prevent or, at least ameliorate the incidence of scaling. If the current is too low, poor water quality is obtained. If the current is too high, the incidence of scaling increases (U.S. Pat. No. 6,365,023). One difficulty in utilizing electrodeionization apparatuses is the deposit of insoluble scale within the cathode compartment primarily due to the presence of calcium, magnesium, and bicarbonate ions in the liquid, which contact the basic environment of the cathode compartment. Scaling can also occur in the concentrating compartments under conditions of high water recovery. In order for calcium carbonate to precipitate in solution the Langelier Saturation Index (LSI) has to be positive. In the cathode compartment the pH can be high enough for the LSI to be positive. The LSI of reverse osmosis product water is always negative. The LSI is even negative in the electrodeionization concentrate stream. Thus, on the basis of consideration of LSI alone, one would not expect the precipitation of calcium carbonate that occurs within concentrating compartments. This phenomenon is instead explainable upon local conditions (U.S. Pat. No. 6,296,751). When the electrodeionization apparatus is in operation, pH near a surface of the anion exchange membrane locally becomes alkaline. CO32− or HCO3− and OH− permeating the anion exchange membrane from the purifying compartments are concentrated near the anion exchange membrane. In addition, hardness contributing polyvalent cations in water in the concentrating compartments are drawn or driven to the anion exchange membrane, so that CO32− or HCO3− and OH− react with Ca2+ to form scales of calcium carbonate on the anion exchange membrane. Build-up of scale can result in an increase in the resistance to the flows of electricity and water through the stack. When scales are formed, the electrical resistance at the area where the scales are formed increases and less electric current flows at that section. At the extreme condition, sufficient current for ion removal cannot be applied within the maximum voltage of the device, and the quality of the treated water deteriorates. Prevention of scale formation typically focuses on removing polyvalent cations from the supply stream by adding water softener. Vendors of the electrodeionization apparatuses suggest that the concentration of calcium in the feed to the system be limited to very low levels; e.g., less than 0.5 ppm (US Filter Literature No. US2006). U.S. Pat. No. 5,308,466 discloses an electrodeionization apparatus utilizing concentrating compartments containing ion exchanging resins. If the concentrating compartments are filled with the ion exchanging resins, the OH− ions permeating through the anion exchange membrane are easy to move in the concentrating compartments, so that the scale is dispersed (U.S. Pat. No. 6,379,518). Acid may be added to convert some of the alkalinity to carbonic acid, and to increase the solubility of carbonate and sulphate salts. The addition of an acidic solution to the concentrate water is disclosed in U.S. Pat. No. 6,274,019. The use of an acidic solution in the concentrate water increases the solubility of the hardness components within the concentrate water and prevents scale formation. The use of effective amount of antiscalant in the concentrating compartments and anode and cathode compartments to inhibit precipitation of scale is disclosed in U.S. Pat. No. 6,056,878. Physical damage can be inflicted on stack components by severe scaling.
U.S. Pat. No. 6,296,751 discloses the use of first and second stages in the electrodeionization apparatus. The purifying compartments of the first stage include only anion exchanging resin or cation exchanging resin material, and thus remove either anions or cations but not the other. The purifying compartments of the second stage receive the purifying compartment effluent from the first stage and include the other type of exchanging resin or a mixed resin material and remove the oppositely charged ions. The concentrate from the first stage is isolated from the second stage to prevent the scaling of sparingly soluble salts in the concentrating compartments.
The use of opposite flow directions for supply stream and concentrate stream is disclosed in U.S. Pat. No. 6,248,226. In conjunction with the use of opposite flow direction, the introduction of a porous diaphragm or ion conducting membrane to divide the concentrating compartments into first and second compartments is disclosed in U.S. Pat. No. 6,149,788 to inhibit scaling. The porous diaphragm or ion conducting membrane effectively eliminates convective transport of scale-forming metallic cations from the cation exchange membrane side of the concentrating compartments to the anion exchange membrane side of the concentrating compartments, thereby inhibiting scale formation on the anion exchange membrane.
Deposits of colloids, organic contaminants, and other impurities on the surface of the membranes and ion exchanging resins generally result in an increase in electrical resistance: this may also result in an increase in the hydraulic resistance in the compartments of the stack and in a decrease in current efficiency (U.S. Pat. No. 5,026,465).
The use of electrodeionization polarity reversal is disclosed in U.S. Pat. No. 5,026,465 to reduce scaling and fouling tendencies by salt precipitates, colloids, organic contaminants, and other impurities. The use of polarity reversal in electrodialysis processes are disclosed in U.S. Pat. Nos. 2,863,813; 3,341,441; 4,115,225; and 4,381,232.